1. Field of the Invention
The present invention relates to a display apparatus using a liquid crystal and, more particularly, to a display apparatus using an active matrix substrate with thin-film switching elements.
2. Related Background Art
In recent years the display apparatus using the liquid crystal are required to achieve higher-definition display images. Among others, display panels of the so-called active matrix type using the thin-film switching elements for drive of pixels are under rapid technological development, because they are relatively easy to achieve increases in the number of pixels and in tone levels as compared with liquid crystal display panels of the other types.
The thin-film switching elements used in the above active matrix type display panels are thin film transistors (TFTs) which normally use amorphous silicon (a-Si) mainly for 5-inch or greater large-scale panels or polycrystal silicon (p-Si) mainly for small-scale panels smaller than 5 inches. FIG. 11 shows a schematic view of a liquid crystal panel using p-Si-TFTs out of them. Vertical shift register 1103 and horizontal shift register 1104 are connected to panel display circuit 1105 including the p-Si-TFTs arrayed in a matrix pattern, as switching elements, and TV image signals transmitted from video signal circuit 1101 are written through the vertical shift register 1103 and horizontal shift register 1104 in the pixels in the display circuit 1105. Numeral 1102 designates a synchronous circuit for achieving timing between the two shift registers 1103, 1104. Recently, there is such a trend that the shift registers 1103, 1104 are made of p-Si-TFTs and that they are integrated in the same panel.
FIG. 10 shows a cross-sectional view of an example of p-Si-TFT. In thin film p-Si on quartz or glass substrate 1001 there are source and drain regions, for example, of n.sup.+ -type diffused layer 1003 and n.sup.- -type diffused layer 1007 and control of on/off is made by applying voltage to gate electrode 1006 made of p-Si through gate insulating layer 1005. The n.sup.- -type diffused layer 1007 is formed especially for the purpose of relaxing the electric field immediately below the gate electrode and in the vicinity of the drain. Numeral 1008 denotes the source/drain electrode, for example, of aluminum, 1010 a layer insulation film, for example, of a silicon oxide film, and 1009 a surface protection film, for example, of a silicon nitride film.
The provision of the above n.sup.- -type diffused layer 1007 relaxes the electric field immediately below the gate near the drain, which decreases the leakage current between the drain and the source, occurring in performing the off operation as being an important characteristic of TFT. There is another method for increasing the thickness of the gate insulating film 1005, thereby decreasing the leakage current and also increasing the withstand voltage of the gate insulating layer.
In the inverse stagger type TFTs, attempts have been made to increase mobility by decreasing surface roughness of the interface between a-Si layer (channel region) and gate insulating layer by anodization, thereby improving characteristics and reliability of transistor (the bulletin of Japanese Laid-open Patent Application No. Hei 8-32083). It is widely known in general that to smooth the interface between the channel region and the gate insulating layer not only in the TFTs but also in bulk Si is very fundamental in improving the characteristics of MOS transistor.
FIG. 12 shows an equivalent circuit in the display section of the liquid crystal panel shown in FIG. 11. A plurality of signal lines 1201a to 1201d and a plurality of scanning lines 1202a to 1202d are connected to the transistors and each scanning line 1202a to 1202d is connected to the gate. A video signal from the signal line 1201a to 1201d is written in pixel electrode 1206. The drain of TFT 1203 is also connected to holding capacitor 1204 for holding a charge written therein for a sufficiently long period, and the other end 1205 of electrode of the capacitor is connected to a common potential to the all pixels or to the pixels in one row.
The conventional active matrix type display apparatus using the p-Si-TFTs as switching elements, however, involved the technological problems described below.
Specifically, it is necessary to provide the long n.sup.- -type diffused layer 1007 as a field relaxing layer in order to sufficiently decrease the leakage current between the drain and the source, but it lowers the driving current of TFT. It also increases the size of element, thereby making the increases in the definition and density difficult. If the thickness of gate insulating layer is increased, rise characteristics of current will become insufficient in the sub-threshold region of TFT.